Preparation of dipotassium monosodium ferricyanide



Feb. 11, 1947. R GRAVELLV 2,415,791

PREPRATION OF DIPOTSSIUM MONOSODIUM FERRIGYANIDE Filed Sept. 19, 1945 lrfv? l ca ca u aber @Mag/:New6 .solar/0N WWW-@j COOZE KE A/a Fe (CA/)C/Pr TAL 5 cfr.: 714m 5 K3 Fe (cn/)6 lNvENToR ROGERS GRAVELL,

my@ uam ATTO R N EY UNITED STATES PATENT PREPARATIN F DIPOTASSIUM MONO-SODIUM FERRICYANIDE Rogers Gravell, Linden, N. J., assigner to AmericanCyanamid Company, New York, N. Y., a corporation of Maine ApplicationSeptember 19, 1945, Serial No. 617,261

GFFICE 2 Claims.

The present invention relates to the preparation of alkali metal ferriand ferrocyanides and specifically to K2Na2Fe(CN)6, K2NaFe('CN s andK3Fe CN s or any of them.

The principal object of the invention is the preparation of one or moreof the above substances from a cheap source of raw materials as acomplete cyclic process without undue waste and without relying upon anyother or related process to take values which would otherwise be lost.

The alkali metal ferri and ferrocyanides are of particular value in theindustry in that they are used extensively as photographic reagents andas intermediates in the production of iron blue complexes for pigmentsor the like.

These complex iron cyanides may be readily prepared from a cyanogencompound such as crude calcium cyanide and ferrous sulfate but due tothe chemical affinity of the Fe(CN) radical for the calcium, difficultyhas heretofore been encountered in forming the alkali metal salt. It hasbeen discovered that by judicious choice of the intermediate alkali oralkaline earth metal ferrocyanide, paying particular attention to itsease of formation and insolubility in the menstruum in which it isformed, a perfect and operative process results.

For instance, calcium ferrocyanide may readily be formed by reacting asolution of calcium cyanide with ferrous sulfate. One of the calciumatoms only may be readily replaced by potassium through reaction withpotassium chloride, for instance. The other calcium atom cannot bereplaced as such with potassium even through large quantities ofpotassium chloride are added at this stage. On the contrary, however,this remaining calcium atom may be readily replaced with sodium as byreaction with soda ash to form K2Na2Fe(CN)s. This material may either berecovered as such in solid form or one of its sodium atoms may be thenknocked out with chlorine to form K2NaFe CN e or this latter materialmay have its single sodium atom replaced with potassium through reactionwith potassium chloride to form KaFe (CNM. While at first blush thiswould appear to be a roundabout method of arriving at an end product,yet by so doing advantage is taken of the ability of one material toreplace another in a ferro or ferricyanide while at the same timechoosing a material to be formed at that stage which is more insolublein the menstruum than the by-products or impurities present there.

The invention further contemplates additional details as hereinaftermore fully described and shown in the drawing in which:

The single figure is a flow sheet showing the preferred method ofcarrying out the invention.

Referring now to the iiow sheet, crude calcium cyanide is placed in aheated extraction vessel to which water or wash liquor from a previousbatch is added, in either case with the addition of an aqueous solutionof ferrous sulfate (FeSO4-7I-I2O), a 30 to 40% solution being preferred.As a result, the following reaction takes place:

Considerable heat is developed as the reaction progresses so that only asmall quantity of steam is required to bring the temperature of theextraction menstruum to about 200 F. which is desired in order to bringabout a complete and adequate extraction. The resulting slurry is lteredon a continuous filter -of the Oliver type or the like and the calciumsulfate'sludge washed with water at about 120 F. The wash water from thecake may be returned to the extraction vessel for an additional cycle.Where desired and in order to improve the speed of ltration, a smallamount of calcium carbonate sludge, obtained later in the process, maybe added to the slurry before filtration although this is not necessary.

This calcium ferrocyanide liquor contains some undesirable sulfate,usually about 0.3% calculated as potassium sulfate. Since the sulfatecontent of such liquors should be reduced to a minimum to produce goodiinal products, the liquors may be passed to a treating tank, sufcientbarium chloride added to precipitate .the sulfate as barium sulfate, andthe sludge removed as by filtering.

The thus purified calcium ferrocyanide is then transferred to aprecipitation tank, heated to boiling with a steam coil and treated withan excess of solid potassium chloride with agitation. The calciumferrocyanide liquor added to the precipitation tank contains about 16%of calcium ferrocyanide and from 6 to 7% sodium chloride, the latterhaving been present as an impurity in the crude calcium cyanide.

-At this stage, there may also be added the mother liquor containingsubstantial quantities of KBFMCNM from a later part of the process thecalcium ferrocyanide liquor used, based on the following reaction:

The calcium ferrocyanide liquor contains reducing substances whichordinarily will reduce most of the ferricyanide to the ferro state. Anyferricyanide not so reduced may be converted by adding amounts of sodiumhydrosulde solution as required. The bulk of the ferrocyanide isprecipitated as the sparingly soluble calcium potassium ferrocyanide andmay be recovered by iiltration, the cake being` Washed with a smallamount of water. The Washings and the filtrate containing the calciumchloride, the excess potassium chloride and the sodium chloride derivedfrom the crude cyanide and recycled mother liquor, are discarded.

The cake of calcium potassium :ferrocyanide is then transferred to aheated mixer equipped with an agitator and slurried With weak liquorobtained from a previous mixer batch. Soda ash solution in slight excessas determined by test, is added according to the following reaction:

3. CaKzFe (CN) 6+Na2CO3= KzNaeFe (CN) s-l-CaCOs and the Whole massheated to boiling by means of a steam coil or the like. The resultingslurry is ltered to recover the precipitated calcium carbonate which iswashed with Water sufficient to yield the volume of weak liquornecessary for preparing the next batch. About of the ferrocyanidepresent in any batch at this stage consists of liquor recirculated inthis way.

The mother liquor from the iltrate contains K2NazFe(CN)s, having acontent of about 31% :calculated as K3Fe CN s and may be used as such inthe arts, or evaporated and the crystals recovered for use..

In the event that it is not desired to recover KzNa2Fe(CN)e, this motherliquor may then be transferred to a chlorinating tank, cooled to about100 F. and oxidized by passing chlorine thereinto at-such a rate thatthe temperature does not exceed 120 F. In the chlorinating tank, thefollowing reaction occurs:

4. KzNasFe (CN e-I-Cl=K2Na-Fe (CN s+NaC1 The end point` is indicated bythe virtual disappearance of ferrocyanide. The K2NaFeCN 6 liquor thusproduced contains the equivalent of about 31% potassium ferricyanideplus about 8% of sodium chloride derived from the ferrocyanide andexcess soda ash. It may be ltered to remove a small amount of insolublematter formed in the previous steps and transferred to a glass linedvacuum evaporator. The solution is heated to about 150 F., the vacuumapplied and the evaporation continued by the introduction of steam tothe jacket at such a rate that the temperature remains at about 160 F.and does not exceed 165 F. These conditions may be readily achieved bymaintaining the vacuum at from about 27 to 29 inches mercury.Evaporation isr continued until about 41% of the total weight ofsolution has been removed as Water, the amount of Water being evaporatedaccording to the exact original concentration so that the mother liquorobtained by cooling the slurry to 60 F. will separate crystals ofK2NaFe(CN)6.

Where necessary, the evaporated liquor and its crystals may betransferred to a magma tank and chlorine added to reoxidize anyferricyanide which may have been reduced to ferrooyanide in theevaporation process. The mother liquor contains about 12% of totalchloride calculated as chlorine. The evaporated liquor, containing anycrystals which have separated during the evaporation, is then cooled,under vacuum to about 60L7 F. and the resulting slurry ltered so as tocollect the crude crystals of K2NaFe(CN)6. They may be further purifiedby recrystallization from Water if desired. The crude crystals containonly about 4% Water and small amounts of chlorides and ferrocyanides asimpurities. When dried, the product contains 102% to 103% offerricyanide calculated as potassium ferricyanide equivalent to 97 to98% calculated as the double salt, with 0.3% to 0.6% chloride calculatedas chlorine, and 0.3% to 0.6% ferrocyanide calculated as :EL Fe(CN) e.The liquor from the formation of crude and purified crystals of K2NaFe(CN)6 may be recycled, as previously mentioned, to the solution ofcalcium ferrocyanide for recovery of the values therein.

Where it is not desired to produce crystals from the K2NaFe(CN)6 liquorbut on the contrary to convert it to K3Fe(CN)6, the crude K2NaFe(CN)eliquor containing about 31% equivalent of KSFMCNM may be treated with0.5 to 0.6 lb. of solid KCl to each pound of equivalent KaFe (CNMpresent according to the following reaction:

5. KzNaFe (CN) e-i-KCl-:KaFe (CN) c +NaC1 It may then be evaporateduntil about 6% of the Weight has been removed as Water. The resultingsolution, including any crystals which have separated during theevaporation, is then cooled to about 60 F. and the crystals of KsFeNh;separated' as by filtration. The product consists essentially ofK3Fe(CN)e With about L1% of Water and small amounts of chlorides andferrocyaniclesas impurities. Actually it contains from `to 101% ofvferricyanide calculated as K3Fe(CN)s, with 0.3 to 0.6% chloridecalculated as chlorine, and 0.3 to 0.6% ferrocyanide calculated asFeNMs.Where desired, the crystals may be further puried by recrystallizationfrom water.

From the above it will be apparent that a self-contained process hasbeen designed for the preparation of K2Na2Fe(CN)e, K2NaFe(CN)s and KaFe(CNM or any of them, recycling values into the process, As a result,improved yields are obtained with consequent reduction in costs overthat heretofore thought possible.

While the invention has beendescribed with particular reference tospecific embodiments, it is to be understood that it is not to belimited thereto but is to beI construed broadly and restricted solely bythe scope of the appended claims.v

I claim:

1. In a method of making K2NaFe CN s by extracting crude CaCN) 2 withWatenadding ferrous sulfate thereto to form` CtagFelDs, adding solid KClto the latter to form crystals of CaKzFeN, separating out the latter,and re acting the same with soda ash solution to form K2Na2Fe(CN)e, theimprovement which includes chlorinating the K2Na2Fe(CN)s to formK2NaFe(CN)s and recovering the latter as crystals.

2. The method of claim 1 in which the solution of KzNaFe (CNM isevaporated until crystals form, and then the magma containing crystalsand mother liquor chlorinated prior to recovery of K2l\TaFe(CN) ecrystals. f

L ROGERS GRAVELL.

